A communication system is well-known, which includes a macro base station providing a macro cell, a home base station providing a home cell that is smaller than the macro cell, and a switching station. The macro base station is also referred to as a Macro Evolved Node B (MeNB), for example. The home base station is also referred to as a Home Evolved Node B (HeNB), for example. The switching station is also referred to as a Mobility Management Entity (MME), for example.
A macro base station may provide multiple cells. For this reason, in a communication system as described above, each cell is identified by a pair of a macro base station identifier identifying a macro base station, and a cell identifier identifying a cell provided by that macro base station. One home base station typically provides a single cell. The number of home base stations is assumed to be larger than that of macro base stations. Hence, each home base station is identified by a home base station identifier having a data length longer than that of a macro base station identifier.
Each macro base station sets up a communication interface (e.g., an S1 interface) between a switching station and that macro base station, using its own macro base station identifier. Each home base station sets up a communication interface between a switching station and that home base station, using its own home base station identifier. The switching station and the respective base stations (each of the macro base station and the home base station) execute communications via the communication interfaces that have been set up.
In the meantime, a relay station (e.g., HeNB GW (Gateway)) is well-known, which relays communications between a home base station and a switching station via a communication interface (refer to Patent Document 1 and Non-Patent Document 1, for example).
One of such relay stations sets up switching station side communication interfaces between the relay station and the switching station, wherein the number of switching station side communication interfaces is smaller than the number of home base stations. As used herein, a switching station side communication interface is a part of a communication interface used by a home base station to communicate with a switching station, wherein the part is between a relay station and a switching station.
This configuration is advantageous in that the number of communication interfaces, that are set up between other stations that directly execute communications with the switching station and the switching station, can be reduced, as compared to a configuration where each of multiple home base stations independently sets up respective communication interfaces with the switching station. This contributes to a reduction in the loads on the switching station.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2012-44320
Non-Patent Document 1: 3rd Generation Partnership Project, Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2 (Release 9), 3GPP TS36.300, December 2011, V9.9.0.
As described above, the switching station side communication interface is set up between the switching station and the relay station. The switching station, however, has no information indicating associations between relay stations and home base stations. Therefore, the switching station that attempts to send information to a destination home base station, cannot find (identify) a relay station that relays communications between the destination home base station and the switching station (the relay station that covers the destination home base station, i.e., covering relay station). Accordingly, for example, when the switching station attempts to send an HO Request requesting a home base station identified by a home base station identifier to execute a handover (HO), that HO request may not arrive at the home base station.
This is particularly probable in the situation where a switching station is configured to identify a destination, solely based on the upper segment of a home base station identifier. As used herein, the upper segment is a part of a home base station identifier. The upper segment has the same data length as that of a macro base station identifier and is located sequentially from the uppermost position. In this particular case, since no other stations are identified by the upper segment of the home base station identifier identifying the destination home base station, it is impossible for the switching station to send any information to the covering relay station.
Another probable situation is in that a switching station is configured to have a function to identify a destination relay station from area information (e.g., a Tracking Area Code (TAC)) associated with a destination of information to be sent. As used herein, area information indicates an area of a home base station. Some switching stations, however, do not have this function. It is impossible for such switching stations to send any information to the covering relay station.
As described above, in the above-described relay station, information may not be conveyed in a reliable manner from a switching station to a home base station.